1. Field of the Invention
This invention relates to an autonomous mobile machine such as a robot or car, as well as a control system and method for navigating a mobile machine to accurately move along a path to be traced while avoiding collisions with obstacles existing around it.
2. Description of Prior Art
This invention employs fuzzy control for navigating mobile machines. Two representative papers on fuzzy control are listed below:
Lofti A. Zadeh, "Fuzzy sets," Information & Control, vol. 8, pp. 338-358, (1965); and PA1 E. H. Mamdani & S. Assilian, "An Experiment in Linguistic Synthesis with a Fuzzy Logic Controller," International Journal of Man-Machine Studies, vol. 7, pp. 1-13 (1974).
Mobile machine navigation is generally categorized into a control procedure for tracing a planned path (path tracing navigation) and a control procedure for avoiding collisions with environmental obstacles (obstacle avoidance navigation). The various schemes for realizing these procedures by using fuzzy control to navigate mobile machines include the following:
(1) Murofushi and Sugano, "Fuzzy Control of a Model Car," Journal of the Robotic Society of Japan, Vol. 6, No. 6, pp. 536-541, Dec. 1988. In order to describe macro actions such as straight movement, right turn, and movement into a garage space, the authors attempt to employ fuzzy control for a certain part of judgement on the status of the vehicle, by measuring distances from walls and the directions of the car itself. They also use fuzzy control for tracing wall surfaces defining the path. (2) Takeuchi, "An Autonomous Fuzzy Mobile Robot," Journal of the Robotic Society of Japan, Vol. &, No. 6, pp. 549-556, Dec. 1988. The author presents a fuzzy control system that receives as inputs the direction of motion P, the path width W, and left and right wheel revolutions, Rl and Rr, obtained through a visual system; and issues as outputs instruction on the speeds of the left and right wheels, Ul and Ur. This system employs a fuzzy system separately for control of direction and of speed increases and decreases. This approach adopts the hallway-following method and avoidance of collisions with stationary obstacles not as wide as the vehicle. (3) Maeda and Takegaki, "Collision Avoidance Control among Moving Obstacles for a Mobile Robot on the Fuzzy Reasoning," Journal of the Robotic Society of Japan, Vol. 6, No. 6, pp. 518-522, 1988. The authors propose a method of avoiding collisions with a single moving obstacle by representing the moving obstacle as a set of points. This approach defines a parameter called "danger of collision," and uses fuzzy control to compute this parameter.
However, these conventional schemes using fuzzy control to navigate a mobile robot have difficulty in coping with a real environment, because a supposed input model may not be actually obtained on account of the limitations of sensor processing, or may lack generality.
Of these schemes, the first is explained as using fuzzy control to judge a situation; however, it uses a threshold value as the final criterion for the judgement, which means that, strictly speaking, it does not use fuzzy control. Specifically, it merely employs partially fuzzy computation to determine the judgement parameters, and the computation is substantially the same linear conversion as in the conventional method. Additionally, this approach does not handle avoidance of collisions with obstacles other than a wall defining the path, and its functions for navigating a mobile robot are therefore insufficient.
In the second scheme, path tracing and obstacle avoidance functions are realized on the basis of the direction of motion P and the path width W with regard to direction control; however, because the supposed path is a hallway, it may not be possible to determine the direction of motion P when the free space is too large. In addition, since the scheme relies on measurement of the direction of motion and the path width by using a visual system, if the lighting is insufficient in the actual environment, necessary parameters cannot be obtained, or even if they are obtained, their accuracies may be low. Further, the need for image processing causes a problem with respect to the response speed.
A common feature of the first two approaches is that the path is defined by obstacles in the form of walls and that path tracing is realized by following the walls. In order to realize such a high level of navigation that the vehicle avoids unexpected obstacles while moving along a path in a free space that has been taught off-line beforehand, path-tracing navigation and obstacle-avoidance navigation must be designed independently.
The third approach is limited to the case in which only one moving obstacle exists, and it attempts to realize obstacle avoidance by using a fuzzy rule that depends on the moving obstacle. Therefore, it cannot operate in an environment where a plurality of stationary obstacles and a moving obstacles exist. Further, although this approach is based on the premise that the position and the relative speed of the supposed moving obstacle are known, the actual moving environment is not simple, and its applicability is therefore limited. It is difficult to obtain such information in a real environment.
There is another known control procedure that designs path-tracing navigation and obstacle-avoiding navigation independently and, while normally navigating a mobile machine to follow a path, switches to obstacle--avoiding navigation in accordance with the status of motion of the mobile machine. However, the reliability of the machine's movements is not ensured because the machine might be suddenly slowed down, or its steering might be abruptly changed.
JA PUPA 2-270006 aims to solve the problem and discloses an autonomous running vehicle that comprises means for obtaining an image of path conditions including a guide line making the path and obstacles, means for permitting a vehicle to run autonomously along the guide line by fuzzy control in response to the image signal, means for discriminating an obstacle in accordance with the image signal, means for setting around the discriminated obstacle a virtual guide line continuously connected to the normal guide line so that the vehicle can move while avoiding the obstacle, and means for permitting the vehicle to run autonomously by fuzzy control along the virtual guide line instead of the normal guide line. This approach involves the following problems:
The first problem is the same as that of the above mentioned second approach in that it needs a visual system. Another problem is that the vehicle avoids the obstacle by tracing the modified path, and no specific control procedure for obstacle avoidance apart from path-tracing navigation has been devised. Therefore, even though it may cope with an obstacle in the form of a point, it cannot deal with moving obstacles or stationary obstacles having a substantial depth.